Introduction to Cocoa
Cocoa is the graphical API for writing GUI programs on Mac OS/X. There are many tutorials available on the web, but most of them want you to use XCode -- Apple's officially endorsed programming environment. Since Tribler is platform-independent this is not really desirable. Hence, this tutorial, which takes a quick tour through Cocoa to get you started without the help of an IDE.
Objective-C
Cocoa is written in Objective-C, which makes it the ideal language for writing Cocoa programs. Objective-C adds OO constructs to C, but is completely downwards compatible. There is an Objective-C beginner's guide that explains everything you need to know on one page. This FAQ could also occasionally prove useful. Objective-C files end in .m, and to compile an Objective-C program using Cocoa, run:
gcc mysource.m -o myprogram.o -framework Cocoa
That's it for Objective-C. Lets do some Cocoa.
Your First Cocoa App
See Your First Cocoa App.
Cocoa Memory Management
Producing a Picture
See Quartz.